Means for converting a mechanical force into an electrical current



April 30, 1963 J. E. FIELD EN 3,037,331

MEANS FOR CONVERTING A CHANICAL FORCE INTO AN ELECTRICAL CURRENT Filed Nov. 23, 1959 2 Sheets-Sheet 1 7 V l'zzvazz/tab April 1963 J E. FIELDEN 3,087,331

MEANS FOR CONiIERTING A MECHANICAL FORCE INTO AN ELECTRICAL CURRENT Filed Nov. 25, 1959 2 Sheets-Sheet 2 United States Patent O 3,087,331 MEANS FOR CONVERTING A MECHANICAL FORCE INTO AN ELECTRICAL CURRENT John Ernest Fielden, Bowdon, England, assignor to Fielden Electronics Limited, Manchester, England, a

British company Filed Nov. 23, 1959, Ser. No. 854,906 Claims priority, application Great Britain July 27, 1959 18 Claims. (Cl. 73-141) This invention relates to transducer means for converting a force into a unidirectional output current.

Known transducer means for this purpose comprise a beam balanceable about an axis, the force being applied to the beam at a point remote from the axis, means for detecting the displacement of the beam :from a predetermined balance position, means controlled by the detecting means providing said output current in dependence upon said displacement and electromagnetic means energised by said output current and operative to restore the beam substantially to the predetermined position after a change in the force.

In one known transducer means the electromagnetic means consists of a coil integral with the beam, energised by the output current and linking the field of a stationary permanent magnet. In another known transducer means the electromagnetic means consists of a plunger-like armature of soft iron movable axially in a cylindrical bore in a stationary soft iron magnetic core, the armature and core being magnetised by a coil energised by the output current. In both these arrangements, as the beam is restored substantially to the predetermined balance position, the coil and armature are respectively in a substantially constant spatial relationship, under equilibrium conditions, with the other elements of the electromagnetic means and the magnitude of the output current is not therefore subject to the variation which would occur if this spatial relationship varied. In the former arrangement the output current is directly proportional to the force and in the latter arrangement it is proportional to the square root of that force.

Because, in these known transducer means, the beam is maintained in substantially a predetermined balance condition it has been usual to support the beam by means of flexible ligaments instead of by means of the less robust agate pivots or the like.

In one known transducer means the beam is of metal and is of laminar form and the displacement detecting means comprises a planar inductor positioned near one end of the beam and lying in a plane parallel to the beam, the arrangement being such that the reactance of the inductor varies with the displacement of the beam from its predetermined balance position. The inductor is connected to the oscillatory circuit of an electronic oscillator and controls the amplitude of oscillation of that oscillator and the output current is derived from a rectifier means having its input circuit coupled to this oscillatory circuit. The output current is thus proportional to the amplitude of oscillation and, therefore, dependent upon the displacement of the beam from its predetermined balance position.

In another known transducer means the displacement detecting means comprises two condenser elements, one being stationary and the other being integral with the beam so that the capacitance between them is dependent upon the displacement of the beam from its predetermined balance condition. This capacitance is connected to the grid circuit of a gas filled triode valve and is effective to control the phase and magnitude of the potential on the grid and, thus, the mean current in the anode circuit of that valve, this anode current being the output current of the transducer means.

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In all the transducers referred to above the force to be transduced is applied to the beam via either a calibration spring or an electromagnetic link. With either of these arrangements movement of the beam is substantially undamped and there is, consequently, a tendency for the beam to oscillate about its predetermined balance position after a change in the applied force or as a result of an external mechanical shock. In order to prevent this it is known to provide an oil-filled damper, the piston and cylinder of the damper being respectively attached to the beam and the stationary structure upon which the beam is supported. The provision of such damper, however, increases the bulk of the arrangement.

The object of the invention is, accordingly, to provide a compact transducer in which unwanted oscillation of the beam is substantially eliminated.

According to one aspect of the invention, in a transducer means for converting a force into an output current comprising a balanceable member to which said force is applied, a movable condenser element integral with said balanceable member, a stationary condenser element, circuit means connected to said condenser elements and providing said output current in dependence upon the capacitance between said condenser elements and electromagnetic means energized by said output current and operative to apply to said balanceable member a balancing force dependent upon said output current to restore said balanceable member substantially to a predetermined balance position, said electromagnetic means comprises a cylindrical permanent magnet the magnetic field of which links an annular coil energised by said output current, said magnet being integral with said balanceable member and movable substantially along the axis of said coil, and said condenser elements are discs positioned perpendicularly to said axis within a cup-shaped body of non-magnetic material charged with electrically insulating liquid whereby said movable condenser element and said cup-shaped body co-act to damp oscillation of said balanceable member.

According to another aspect of the invention, in a transducer means for converting a force into an output current comprising a balanceable member to which said force is applied, a movable condenser element integral with said balanceable member, a stationary condenser element, circuit means connected to said condenser elements and providing said output current in dependence upon. the capacitance between said condenser elements and electromagnetic means energized by said output current and operative to apply to said balanceable member a balancing force dependent upon said output current to restore said balanceable member substantially to a predetermined balance position, said electromagnetic means comprises a cylindrical permanent magnet the magnetic field of which links an annular coil energized by said output current, said magnet being integral with said balanceable member and movable substantially along the axis of said coil, said stationary condenser element is a disc positioned perpendicularly to said axis within a cup-shaped body of non-magnetic material, said magnet is said movable condenser element and is positioned within said cupshaped body and said cup-shaped body is charged with electrically insulating liquid whereby said magnet and said cup-shaped body co-act to damp oscillation of said balanceable member.

Refer-ring by way of example to the accompanying drawings:

FIGURE 1 is a partly schematic and partly sectional view of one embodiment of the invention,

FIGURE 2 is a partly sectional view of another embodiment of the invention, suitable for measuring small differences in gas pressures,

FIGURE 3 is a sectional view of part of an embodiment of the invention for use in any position without loss of liquid,

FIGURE 4 is a vertical sectional view of part of an embodiment of the invention, suitable for measuring the dilference between two high pressures and,

FIGURE shows an embodiment of the invention, using a vertical shaft as the balanceable member.

In the figures corresponding elements are indicated by the same reference numerals.

In the arrangement shown partly schematically and partly in section in FIGURE 1, a coil 1 is positioned with its axis vertical and axially aligned with it is a permanent magnet 2 mounted upon a threaded shaft 3, the magnet being positioned so that its mid horizontal plane is slightly below the mid horizontal plane of coil 1. The upper end of the shaft 3 is fixed to one end of a beam 4 by means of two lock-nuts 3a, this beam being pivotally supported by means of crossed flexures 5 fixed to a pivot block 9 which is, in turn, fixed to a base 13. The force to be transduced is applied in an upward direction to the beam 4 via an anchor plate 14. Soldered to the lower end of the shaft 3 is a circular horizontally disposed condenser plate 6. The shaft 3 passes through a smooth axial bore 2a in the magnet 2 and the latter is retained with its lower end face abutting the upper face of the plate 6 by means of the nut 3b.

The coil 1 is wound upon a former of electrically insulating non-magnetic material. Positioned beneath the former is a cylindrical tube 8, also of electrically insulating non-magnetic material, which surrounds the condenser plate 6 and which is joined to the former 15 by a fluid-tight joint formed by pressing a cylindrical flange 16 on the under side of the former 15 into the bore of the tube 8. The tube 8 and former 15 together form a generally cup-shaped body, indicated generally by the numeral 17, which is closed at its lower end and Which is fixed to the base 13 by a nut 13a screwed onto the shank 19. of a second circular horizontally disposed condenser plate 7.

There is a possibility that inaccuracies in the threads of the lock-nuts 3a and of the shaft 3 may cause the axis of the shaft 3 to be slightly displaced from the verticalwhen the nuts 3a are tightened. If the plate 6 were of a uniform thickness such a displacement of the shaft 3 would result in the gap between the plates 6 and 7 being non-uniform. In order to eliminate this error the lower face of the plate 6 is of spherical form, being a portion of a sphere described about the junctions of the shaft 3 and beam 4.

The base 13 is electrically earthed and the condenser plate 6, magnet 2, shaft 3 and beam 4 are electrically connected to the base 13 via the crossed flexures 5. The condenser plate 7 is insulated from the base 13 and is connected via a co-axial cable to a capacitancesensitive relay 11 which provides, in known manner, an output current dependent upon the capacitance between the condenser plates 6 and 7. The output current from the relay 11 is passed through the coil 1 to output terminals 12 and the coil 1 is so connected that the resultant magnetic field which it produces repels the magnet 2 in a downward direction. The arrangement is so adjusted that the output current of the relay 11 is zero when the mechanical force to be transduced is zero, the magnet 2. then occupying a predetermined position and the displacement between the condenser plates 6 and 7 having a predetermined magnitude, a biassing force being applied to the beam 4 by means of a counterweight (not shown) or the like to maintain the displacement between the condenser plates 6 and 7 at this predetermined value when the force to be transduced is zero.

The anchor plate 14 projects upwards through a longitudinal slot (not shown) in the beam 4 and has screwthreaded engagement with a lead screw 21. This lead screw 21 is provided at one end with a knurled knob 22 and it is biassed towards the right, as shown in the figure, by means of a spring 21), which bears against the left hand face of the knob 19. The spring 20 enables the position of the anchor plate 14 to be maintained accurately after adjustment by rotation of the knob 22.

In operation, the application of the mechanical force to be transduced in an upward direction to the beam 4 via the anchor plate 14 causes the plate 6 to move upwards and the capacity between the plates 6 and 7 to decrease. The resulting output current provided by the relay 11 causes a magnetic force to be applied to the magnet 2 in a downward direction with the result that the displacement between the condenser plates 6 and 7 is reduced to a magnitude suflicient to maintain the output current at a value sufiicient to maintain the displacement at that magnitude. The sensitivity of the relay 11 is sufiicient to ensure that maximum output current is obtained for a very small change in the displacement between the plates 6 and 7 from the predetermined value with the result that this displacement is maintained at substantially the predetermined value and the magnet 2 is substantially maintained in its predetermined position.

The bore of the tube 3 and the diameter of the condenser plate 6 are such that, when the tube 8 is charged with a suitable electrically insulating liquid 18 preferably a light oil, at least to a level higher than that of the movable condenser plate 6, the tube 8 and condenser plate 6 co-operate in the manner of a dashpot to damp movement of the magnet 2 and beam 4.

The direction of the mechanical applied force to the beam 4 may be reversed provided that the direction of the magnetic force applied to the beam 4 is also reversed.

In a modified arrangement the condenser plate 6 is eliminated, the lower surface of the magnet 2 being used instead. The magnet is then also used as a. dashpot plunger, the cup-shaped body 17 being then charged with liquid 18 to a level above the top of the magnet 2.

In the arrangement of which part is shown in section in FIGURE 2., which is suitable for measuring small differences of gas pressure, a beam 4 is pivotally supported upon a pillar 23 by means of crossed fiexures 5 fixed to the top of the pillar 23 and a pivot block 9, the pivot block 9 being fixed to the vertical side of the beam 4. A second pivot block is fixed to the side of the beam which is hidden in,the figure and this also is attached to the top of the pillar 23 by additional crossed fiexures, also not shown. Fixed to one end of the beam 4 is a shaft 3 carrying a magnet 2 and a condenser plate 6 which move within a generally cup-shaped body 17. The shaft 3, magnet 2, condenser plate 6 and body 17 form part of an arrangement similar to that described above with reference to FIGURE 1, the arrangement providing an output current proportional to the force applied to the upper end of the shaft 3. Fixed to the other end of the beam 4 is a vertical tube 27 of uniform bore, the upper end of the tube being closed and the lower end extending below the level of the top of the body 17.

The body 17 and the pillar 23 are fixed to the bottom of an hermetically sealed chamber 24 provided with a port 25 positioned in one side of the chamber 24 above the level of the top of the body 17 and a port 26 positioned in the bottom of the chamber 24 vertically beneath the open lower end of the tube 27. A tube 28 of uniform bore extends vertically upwards from the port 26 inside the tube 27, substantially to the top of the tube 27.

The chamber 24 is "charged with a suitable liquid 18, preferably a light electrically insulating oil, to a level above the top of the body 17 but below the port 25.

When the pressures applied to the ports 25 and 26 are equal the levels of liquid inside and outside the tube 28 are the same. The arrangement is so adjusted that the output current is then zero. When the pressure applied to the port 26 is less than that applied to the port 25 the level of liquid in the tube 27 rises by an amount proportional to the difference between these pressures. The volume of liquid in the chamber 24 is very much greater than the volume of liquid which rises in the tube 27 so that no substantial change occurs in the level of liquid in the chamber 24 as the level rises or falls in the tube 27. Changes in the apparent weight of the tube 27 due to change in liquid level are thereby rendered negligible.

It can be shown that the downward thrust exerted upon tube 27 due to the difference between the pressures applied to ports and 26 is proportional to this pressure difference. This downward thrust tends to cause the right hand end of the beam 4, as shown in the figure, to move downwards but this downward movement is opposed, in a manner similar to that described above with reference to FIGURE 1, by the magnetic force applied in a downward direction to the other end of the beam by the interaction of the magnet 2 and the coil 3. The output current required to maintain the beam 4 in substantially its original position is thus proportional to the difference in the pressures applied to the ports 25 and 26.

If a non-linear relationship between output current and pressure difference is desired the tube 27 may be replaced by a vessel so shaped that its horizontal internal section varies with height in a suitable manner.

The arrangement described above with reference to FIGURE 2 may be modified, the modifications corresponding to the modifications described above with reference to FIGURE 1. For example, the directions of the magnetic force applied to the beam 4 must be reversed if the pressure applied to the port 26 exceeds that applied to the port 25. I

In FIGURE 3 there is shown, in section, a portion of an arrangement which may be used in any position without loss of liquid from the dashpot. The arrangement is generally similar to that described above with reference to FIGURE 1, but the cup-shaped body 17 is closed by means of a flabby diaphragm 29, of neoprene or the like, having an enlarged and relatively stiff perimeter ring so dimensioned as to be a push fit within the open end of the cup-shaped body 17. The shaft passes through a hole in the centre of the diaphragm, the hole being closed in a liquid-tight manner and the shaft 3 being fixed to the centre of the diaphragm by means of nuts 31 and washers 32. The body 17 is filled with light insulating oil through a bore in the shank 19 of a condenser plate 7, the bore being closed by a screw 34 and washer 35.

T he use of the flabby diaphragm 29 permits slight longitudinal movement of the shaft 3 without alteration of the volume enclosed within the body 17, this volume necessarily remaining constant under constant temperature conditions due to its being fully charged with oil. Furthermore, it permits the volume of oil enclosed Within the body 17 to change with change in temperature without causing the diaphragm to transmit a substantial force to the shaft 3 in the longitudinal direction.

FIGURE 4 is a vertical sectional view of part of an arrangement which is suitable for measuring the difference between two high pressures, for example, the differential pressure occurring across an orifice plate in a high pressure steam line. In this arrangement the beam 4 is supported at one end from a pivot block 9 by means of crossed flexures 5 and it carries at its other end a magnet 2 which moves substantially axially within a cup-shaped body 17 0f non-magnetic material. On that end of the magnet 2 which is remote from the beam 4 is fixed a circular condenser plate 6 and adjacent to the condenser plate 6 is an insulated circular condenser plate 7 the shank 19 of which projects through the end of the body 17 A coil 1 encircles the body 17, the coil being wound in a former 15 so dimensioned that its bore is a push fit over the body 17.

- The body 17 forms part of a housing indicated generally by the reference numeral 36, the remainder of the housing being formed by bolting together two co-operating casing members 37 and 38 the housing 36 being sealed by a ring 39 of resilient material. The pivot block 9 is bolted to the casing member 37. The housing 36 is completely filled with incompressible liquid, preferably a light electrically insulating oil, via the ports 46, both of which are subsequently closed by plugs 47. The interior of the housing 36 is so shaped that the volume of liquid contained in it is a minimum, thereby reducing the magnitude of errors resulting from change in the volume of the liquid with change of temperature.

The casing member 38 is provided with a port 25 communicating with a chamber 40 within the casing member 38. Exposed on one side to the pressure in chamber 40 and on the other side to the pressure within the housing 36 is a flexible diaphragm 41 through which the pressure in chamber 40 is transmitted to the interior of housing 36 via a port 40a. Preferably a ported backing plate 48 is provided on each side of the flexible diaphragm 41 for the purpose of preventing the flexible diaphragm being damaged as a result of the pressures at port 25 and in the interior of housing 36 differing by too great a margin.

The casing member 37 is provided with a port 26 communicating with a chamber 43. Within the casing member 37 and exposed on one side to the pressure in chamber 43 and on the other side to the pressure within the housing 36 is a flexible diaphragm 44 movement of which in response to changes in pressure in chamber 43, is transmitted to the beam 4 via the diaphragm follower 45. A ported backing plate 49 is provided adjacent to the external surface of the diaphragm 44 in order to prevent the diaphragm being damaged as a result of pressure within the housing 36 exceeding by too great a margin the pressure applied to port 26. T o prevent the diaphragm 44 being damaged by too great a pressure difference in the reverse direction an O-ring 50 is provided, this O-ring being mounted upon a block 51 fixed to the beam 4 and being arranged to sea]. port 40a when the pressure applied to port 26 greatly exceeds the pressure within the housing 36.

The housing 36 is earthed and, as it is electrically connected to the condenser plate 6 via the pivot block 9, crossed flexures 5, beam 4 and magnet 2, the condenser plate 6 is also earthed. The housing 36 and the shank 19 of the insulated condenser plate 7 are respectively connected by means not shown to the input terminals of a relay, also not shown, similar to the relay 11 referred to above with reference to FIGURE 1, this relay providing an output current dependent upon the displacement of the condenser plate 7 from the condenser plate 6. The arrangement is such that when the difference between the pressures at ports 25 and 26 is zero the magnet 2 occupies a predetermined position and the displacement between the condenser plates 6 and 7 has a predetermined value. The output current from the relay passes through coil 1 and the arrangement operates in a manner similar to that described above with reference to FIGURE 1, movement of the beam 4 being opposed by the magnetic force applied to the magnet 2 due to energisation of the coil 1, the magnet 2 being maintained substantially in its predetermined position and the relay providing an output current substantially proportional to the mechanical force applied to the beam 4 via the diaphragm 44. As this force is proportional to the pressure difference across the diaphragm 44 the output current is substantially proportional to this pressure difference.

In operation, the pressure applied to port 25 is normally lower than the pressure applied to port 26. The liquid contained on the housing 36 is subject to the pressure at port 25 and the Whole of the pressure difference between the pressures applied to ports 25 and 26 is applied to the flexible diaphragm 44. The resulting thrust developed on the diaphragm 44 is opposed by the reaction between the coil 1 and magnet 2 amplified by the mechanical advantage of the beam 4.

The difference between the internal diameter of the body 17 and the diameter of the condenser plate 6 is sufficiently small to cause the flow of liquid from one side to the other of plate 6 during movement of that plate to be sufficiently restricted to provide adequate damping of that movement.

The housing 36 is mounted upon a pivot (not shown) fixed to a mounting bracket 52 and its position relative to the bracket 52 is adjustable by manual adjustment of the nut 53. A helical spring 54 is interposed between the housing 36 and the bracket 52, the spring 54 being compressed and thereby ensuring that pivotal movement of the housing 36 occurs only as a result of adjustment of the nut 53. As the position of the magnet 2 relative to the coil 1 is largely dependent upon the direction of the force of gravity, adjustment of the nut 53 is a convenient method of adjusting the output current to zero when the pressure at ports 25 and 26 are equal.

The coil 1 and shank 19 are protected from atmospheric contamination by a shield 55 the wall of which is pierced by a threaded hole 56 into which a metal conduit may be screwed. Connections to the coil 1 and shank 19 from the relay are brought through this conduit into the shield 55.

The arrangement described above with reference to FIGURE 4 may be modified, the modifications corresponding to the modifications described above with reference to FIGURE 1.

In the arrangement of which part is shown in FIG- URE 5, instead of using a beam as the balanceable member, as in the arrangements described above, a resiliently supported vertical shaft is used. As shown in FIGURE 5 a coil 1 is positioned with its axis vertical and axially aligned with it is a permanent magnet 2 mounted upon a shaft 3, the magnet 2 being positioned so thatits mid horizontal plane is slightly below the mid horizontal plane of the coil 1. The upper end of the shaft 3 is attached to the lower end of a helical spring 42 to the upper end of which is applied the force to be, transduced. Mounted upon the lower end of the shaft 3 is a circular condenser plate 6, the plane of the condenser plate being horizontal. A cylindrical tube 8 of non-magnetic material surrounds the magnet 2 and condenser plate 6 and is arranged with its axis vertical, the outside diameter of the tube 8 being smaller than the internal diameter of the coil 1 and the inside diameter of the tube 8 being larger than the outside diameters of the magnet 2 and condenser plate 6. The coil 1 may be wound directly on the tube 8 or it may, alternatively, be wound on a former which is supported by the tube 8. The tube 8 is closed at its lower end and supported from an insulating bushing 59 in the lower end of the tube 8 is a second circular condenser plate 7 arranged with its plane horizontal. A group of centralising springs 57 of non-magnetic material are positioned in the annular space between the bore of the tube 8 and that portion of the shaft 3 extending from the magnet 2 to the condenser plate 6 for the purpose of maintaining the shaft 3 aXially aligned within the tube 8. The centralising springs 57 are mounted upon an annular ring 58, which is a push-fit within the uniform bore of the tube 8.

The condenser plate 6, magnet 2, shaft 3 and spring 42 are electrically earthed and the condenser plate 7 is connected via the bushing 59 to a capacitance-sensitive relay (not shown) providing an output current dependent upon the capacitance between the condenser plates 6 and 7. Coil, 1 is energised by this output current and is so arranged that the resultant magnetic field set up by the coil 1 repels the magnet 2. in a downward direction. The arrangement is so adjusted that the output current is zero when the displacement between the condenser plates 6 and 7 has a predetermined magnitude, this displacement being maintained at this value when the force to be transduced. is zero by the application of a biasing force in an upward direction to the upper end of the spring 42 for the purpose of balancing the gravitational force acting upon the movable members viz: the spring 42, shaft 3,

magnet 2 and condenser plate 6. In operation the mechanical force to be transduced' is applied in an upward direction to the upper end of the spring 42 causing the movable elements to move upwards and thereby causing a decrease in the capacitance between the plates 6 and 7. The resulting energization of the coil 1 causes a magnetic force to be applied to the magnet 2 in a downward direction the magnitude of this magnetic force being such as to maintain the displacement between the condenser plates 6 and 7 at substantially the predetermined magnitude.

The tube 8 is charged with a suitable electrically insulating liquid 18 at least to a level higher than that of the movable condenser plate 6 and the tube 8 and condenser plate 6 co-operate to damp vibrations of the movable members.

'By arranging that the magnet 2 is repelled by the magnetic field of the coil 1 the magnetic force applied to the magnet 2 when the coil 1 is energised tends to maintain the magnet 2 axially aligned with respect to the coil 1. I

.The magnet 2 may, alternatively, be positioned with its mid horizontal plane above the mid horizontal plane of the coil 1, the magnet 2 being repelled upwards when coil 1 is energised and the mechanical'force being applied in a downward direction -to the spring 42.

The magnet 2 may, alternatively, be attracted by the magnetic field of the coil 1 but the reaction between the magnetic field of the magnet 2' and the magnetic fieldi of the coil 1 does not then tend to maintain the magnet 12 axially aligned with respect to the coil 11.

The magnet 2 may, alternatively, be mounted at the lower end of the shaft 3 and may itself act as a condenser plate co-operating with the condenser plate 7 to form the capacitance which controls the output current. The internal diameter of the tube 8 and the external diameter of the magnet 2 are then so chosen that the magnet 2 acts as a dashpot plunger within the tube 8. modification the group of centralising springs 57 is positioned above the magnet 2.

In the arrangement shown in FIGURE 5 the axis of the coil 1 may depart from the vertical provided there is no possibility of the liquid escaping from the tube 8 and the phrase approximately vertical as used herein.

in relation to the alignment of the axis of the coil 1 is to be understood as including all positions of that axis in which there is no possibility ofthe liquid escaping.

What I claim is:

1. A transducer means for converting a force into an, output current comprising a balanceable member to which said force is applied, a condenser element movable with said balanceable member, a stationarycondenser element in opposed relation to the movable condenser element, circuit means connected to said condenser elements and; providing said output current in dependence upon the capacitance between said condenser elements, electromagnetic means energized by said output current and operative to apply to said balanceable member a balancing force dependent upon said output current to restoresaid balanceable member substantially to a predetermined balance position, said electromagnetic means comprising an annular coil energised by said output current, and a. magnetized core within the coilsecured to said balanceable member and movable substantially along the axis of said coil, said condenser elements including discs concentric with said axis, a cup-shaped body of non-magnetic material charged with electrically insulating liquid within which the discs are located whereby said movable condenser element and said cup-shaped bodyco-act to damp oscillations of said balanceable member.

2. A means for providing anelectric current proportional to the differences between two pressures comprising a transducer means according to claim 1 wherein said balanceable member is a pendulum supported adjacent its upper end about a horizontal support axis and en- In this.

closed within a housing formed in part of a flexible diaphragm and said cup-shaped body, said core being rigidly attached to the lower end of said pendulum, the interior of said housing being fully charged with insulating liquid and being exposed to one said pressure, the external surface of said flexible diaphragm being exposed to the other said pressure and the thrust developed on said diaphragm being transmitted by a mechanical linkage to a point on said pendulum remote from said support axis.

3. A means according to claim 2 wherein said housing is formed in part by a second flexible diaphragm, the external surface of which is exposed to said one pressure.

4. A means according to claim 2 wherein said housing is pivotally mounted upon a supporting bracket, means being provided for adjusting the angular position of said housing relative to said bracket.

5. A transducer means according to claim 1 wherein said balanceable member is a resiliently supported shaft axially aligned with said coil and restrained to move along said axis.

6. A transducer means according to claim 1 wherein said core is repelled by said coil when said coil is energised.

7. A transducer means according to claim 1 wherein said coil is wound on a former which forms part of said cup-shaped body.

8. A transducer means according to claim 1 wherein said cup-shaped body is closed by a flabby diaphragm.

9. A transducer means for converting a force into an output current comprising a balanceable member to which said force is applied, a condenser'element movable with said balanceable member, a stationary condenser element in opposed relation to the movable condenser element, circuit means connected to said condenser elements and providing said output current in dependence upon the capacitance between said condenser elements, electromagnetic means energized by said output current and operative to apply to said balanceable member a balancing force dependent upon said output current to restore said balanceable member substantially to a predetermined balance position, said electromagnetic means comprising an annular coil energised by said output current and a magnetized core within the coil, secured to said balanceable member and movable substantially along the axis of said coil, said stationary condenser element comprising a disc concentric with said axis and said core forming the movable condenser element, a cup-shaped body of non-magnetic material, said condenser elements being located within said cup-shaped body, said cup-shaped body being charged with electrically insulating liquid whereby said core and said cup-shaped body co-act to damp oscillations of said balanceable member.

l0. A means for providing an electric current proportional to the difference between two pressures comprising a transducer means according to claim 9 wherein said balanceable member is a pendulum supported adjacent its upper end about a horizontal support axis and enclosed within a housing formed in part of a flexible diaphragm and said cup-shaped body, said core being rigidly attached to the lower end of said pendulum, the interior of said housing being fully charged with insulating liquid and being exposed to one said pressure, the external surface of said flexible diaphragm being exposed to the other said pressure and the thrust developed on said diaphragm being transmitted by a mechanical linkage to a point on said pendulum remote from said support axis.

11. A means according to claim 10, wherein said housing is formed in part by a second flexible diaphragm, the external surface of which is exposed to said one pressure.

'12. A means according to claim 10, wherein said housing is pivotally mounted upon a supporting bracket, means being provided for adjusting the angular position of said housing relative to said bracket to adjust said balanceable member to said predetermined balance position when said pressure dilierence is zero.

13. A transducer means according to claim 9, wherein said balanceable member is a resiliently supported shaft axially aligned with said coil and restrained to move along said axis.

14. A transducer means according to claim 9, wherein said core is repelled by said coil when said coil is energized.

15. A transducer means according to claim 9, wherein said coil is wound on a former which forms part of said cup-shaped body.

16. A transducer means according to claim 9, wherein said cup-shaped body is closed by a flabby diaphragm.

17. A transducer means for converting a force into an output current comprising a balanceable member to which said force is applied, a movable condenser element connected with said balanceable member, a stationary condenser element, circuit means connected to said condenser elements and providing said output current in dependence upon the capacitance between said condenser elements and electromagnetic means energized by said output current and operative to apply to said balanceable member a balancing force dependent upon said output current to restore said balanceable member substantially to a predetermined balance position wherein said electromagnetic means comprises a cylindrical permanent magnet the magnetic field of which links an annular coil energised by said output current, said magnet being connected with said balanceable member and movable substantially along the axis of said coil, and said condenser elements are discs positioned perpendicularly to said axis within a cupshaped body of non-magnetic material charged with electrically insulating liquid whereby said movable condenser element and said cup-shaped body co-act to damp oscillations of said balanceable member, and wherein said balanceable member is a beam pivotally supported about a horizontal axis and having a laminar portion remote from said axis and said movable condenser disc is integral with and perpendicular to the lower end of a shaft having a threaded portion extending through a hole in said laminar portion, two nuts on said threaded portion abutting the opposite faces respectively of said laminar portion and thereby rigidly attaching said shaft to said beam, the lower face of said movable con denser disc being a portion of a sphere described about the intersection of the axis of said shaft and the plane of said laminar portion.

18. A means for providing an electric current dependent upon the difference between two gaseous pressures comprising transducer means according to claim 17 wherein a vertical tube, closed at its top, is rigidly attached to said beam at a point remote from said horizontal axis, said transducer means and said tube are enclosed in an hermetically sealed housing charged with an insulating liquid to a level above both the lower end of said tube and the upper end of said cup-shaped body, a conduit supported from the interior of said housing extending within said tube throughout the length thereof and opening to the interior of said tube adjacent to the top thereof, said pressures being applied to the interior of said housing and said conduit respectively.

References Cited in the file of this patent UNITED STATES PATENTS 2,346,593 Lindblad et al Apr. 11, 1944 2,371,040 Fisher et al. Mar. 6, 1945 2,484,541 Allwein Oct. 11, 1949 2,988,921 Rubin June 20, 1961 

1. A TRANSDUCER MEANS FOR CONVERTING A FORCE INTO AN OUTPUT CURRENT COMPRISING A BALANCEABLE MEMBER TO WHICH SAID FORCE IS APPLIED, A CONDENSER ELEMENT MOVABLE WITH SAID BALANCEABLE MEMBER, A STATIONARY CONDENSER ELEMENT IN OPPOSED RELATION TO THE MOVABLE CONDENSER ELEMENT, CIRCUIT MEANS CONNECTED TO SAID CONDENSER ELEMENTS AND PROVIDING SAID OUTPUT CURRENT IN DEPENDENCE UPON THE CAPACITANCE BETWEEN SAID CONDENSER ELEMENTS, ELECTROMAGNETIC MEANS ENERGIZED BY SAID OUTPUT CURRENT AND OPERATIVE TO APPLY TO SAID BALANCEABLE MEMBER A BALANCING FORCE DEPENDENT UPON SAID OUTPUT CURRENT TO RESTORE SAID BALANCEABLE MEMBER SUBSTANTIALLY TO A PREDETERMINED BALANCE POSITION, SAID ELECTROMAGNETIC MEANS COMPRISING AN ANNULAR COIL ENERGIZED BY SAID OUTPUT CURRENT, AND A MAGNETIZED CORE WITHIN THE COIL SECURED TO SAID BALANCEABLE MEMBER AND MOVABLE SUBSTANTIALLY ALONG THE AXIS OF SAID COIL, SAID CONDENSER ELEMENTS INCLUDING DISCS CONCENTRIC WITH SAID AXIS, A CUP-SHAPED BODY OF NON-MAGNETIC MATERIAL CHARGED WITH ELECTRICALLY INSULATING LIQUID WITHIN WHICH THE DISCS ARE LOCATED WHEREBY SAID MOVABLE CONDENSER ELEMENT AND SAID CUP-SHAPED BODY CO-ACT TO DAMP OSCILLATIONS OF SAID BALANCEABLE MEMBER. 